1. Field of the Invention
This invention relates to ion selective electrodes, and more particularly to an electrode formed by coating a conductor with an ion exchange material.
2. Description of the Prior Art
In general, there are two types of electrodes in conventional use. The first, and simplest, are those prepared with fine metallic wires, such as Ag and Cu, which respond to activity changes in Ag.sup.+ or Cu.sup.++, respectively. Unfortunately, these types of electrodes are only available for a very limited number of ions.
A wider variety of ions give potentiometric responses to a second type of electrode, in which a metal wire is coated with a poorly soluble salt of the metal, such as an Ag/AgCl electrode, which responds to activity changes in Cl.sup.-.
Membrane electrodes, which are highly selective to various ions, are well known in the prior art. Usually, these types of electrodes consist of a conductive electrode immersed in an aqueous reference solution, which is in contact with a "membrane" or permeable barrier. An interface is formed between the permeable barrier and the test solution which enables potentiometric measurements of the test solution. Probably the oldest and most established example of a membrane electrode is the glass electrode, universally used for pH measurements. More recently available (Orion and Corning) membrane electrodes involve a layer of a liquid which is immiscible with water as a "membrane."
There are many difficulties attendant to the use of commercially available membrane electrodes. For one, they are rather bulky in size, and, hence, are not completely adaptable for many applications. In particular, a need exists for a miniaturized version of the membrane electrode for possible use in ultramicroanalytical situations, including possibly intracellular measurements. Another difficulty with those commercially available electrodes is that their use of a free flowing internal reference solution limits their physical orientation; that is, they cannot be used in an inverted position, since the reference solution will spill out of the electrode.
Another difficulty with the conventional membrane electrodes is that they are somewhat complex in structure and hence are very expensive to produce and frequently expensive to maintain and to protect from breakage.
One approach to production of a solid state membrane type electrode, which might be adaptable to miniaturization, has been disclosed by Hirata et al. in Talanta, 1970, Vol. 17, Page 883, which disclosed the feasibility of affixing a membrane directly to a non-reactive metal wire, thus eliminating the internal reference solution. According to that disclosure, a Cu.sup.++ selective electrode was produced by attaching a Cu.sub.2 S-impregnated silicone rubber or epoxy resin membrane to either a copper plate or a platinum wire. Although that electrode is considerably less bulky than the prior art membrane electrodes, it suffers the disadvantage that poor results are obtained unless the membrane is loaded with sufficient Cu.sub.2 S, so that the Cu.sub.2 S particles are in sufficient mutual contact to establish lines of conductivity from the surface of the membrane to the non-reactive metal. This type of electrode, therefore, might be considered to be merely a modification of a Cu/Cu.sub.2 S precipitate type electrode, wherein a layer of Cu is in contact with a layer of Cu.sub.2 S.
Moreover, the Hirata et al. electrode is limited in the type of salts which can be incorporated therein. As is well known, the usefulness of a particular electrode depends upon the particular potentiometrically responsive ion contained in the system. For instance, a Cu/Cu.sub.2 S electrode can only be used to measure activity changes in Cu ions or sulfide ions. When other measurements or titrations are required, different electrode systems are necessary. The Hirata et al. electrode is therefore limited to those systems for which a suitable combination of metal-metal salts can be formed; that is, given the type of ion, a suitable slightly soluble salt must be found to form the metal/metal salt precipitate type system. Such suitable salts, however, are not always available.
Another approach to solid state membrane type electrodes has been disclosed by M. R. Thompson, Journal of Research of the National Bureau of Standards 9, 833 (1932), in which a thin metal film is plated onto the inside of an otherwise conventional glass, as a replacement for the internal reference solution. That system is similarly limited as the Hirata et al. system, in the variety of ion selective electrodes which can be formed.
It would be desirable, therefore, to provide an electrode of low bulk, which can be easily miniaturized, which has a high degree of uniformity in response, is easily manufacturable and which can easily be adapted for potentiometric measurement of a wide variety of ions.